Transducer for measuring environmental parameters

ABSTRACT

Apparatus, methods, and other embodiments associated with measuring environmental parameters are described herein. In one embodiment, a transducer comprises a tube, an elongated member, a first reflective surface, a second reflective surface, and an optical fiber. The tube has a first end and a second end, and the elongated member also has a first end and a second end, with the first end of the elongated member secured to the tube. The second reflective surface is secured to the second end of the elongated member, and the first reflective surface is spaced apart from the second reflective surface and secured to the second end of the tube. The optical fiber is positioned to direct light towards the first and second reflective surfaces and to collect the reflected light from these two surfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/011,057 entitled “TRANSDUCER FOR MEASURING ENVIRONMENTALPARAMATERS” filed on Jan. 24, 2008, issued as U.S. Pat. No. 7,787,128,which claims priority from U.S. Provisional Patent Application No.60/897,093 entitled “PRESSURE TRANSDUCER FOR USE AT HIGH PRESSURES”filed on Jan. 24, 2007, which are hereby incorporated by reference intheir entirety.

FIELD OF THE INVENTION

The present invention generally relates to pressure and temperaturetransducers and more particularly to small diameter transducers for usein measuring high pressures at high temperatures.

BACKGROUND OF THE INVENTION

Apparatus for measuring high pressures at low temperatures are known inthe prior art. For example, measurement of pressure at temperaturesbelow 350° F. is frequently accomplished with pressure transducers thatposition a large diameter diaphragm such that the diaphragm is exposedto the pressure to be measured. Such diaphragms are typically thin orcorrugated and have relatively large diameters. The diaphragms are oftenrigidly clamped or welded in place at the perimeter of the diaphragmsuch that the central portion of the diaphragm is compliant and deflectsproportionally in response to pressure. The amount of deflection of thediaphragm may be used to calculate the pressure.

Pressure measurement techniques for relatively moderate temperatures arewell known in the prior art. As the range of pressure to be measured ina family of transducers increases, it may be necessary to increase thethickness of the diaphragm to assure all of the transducers in thefamily deflect approximately the same amount when each transducer in thefamily is subjected to 100% of its pressure rating. All other factorsbeing equal, the stress on the diaphragm increases as the thickness ofthe diaphragm increases or as the diameter of the diaphragm decreases.So, for a given diameter of transducer, there exists a maximum pressurerating above which the stresses in the diaphragm exceed the allowablestresses for the material, and the transducer begins to yield and deformplastically. Yielding and plastic deformation occur because the bendingstresses around the circumference of the diaphragm exceeds the elasticstrength of the diaphragm material. Such yielding and plasticdeformation results in a loss of repeatability and stability of thetransducer.

Pressure measurement at high temperatures creates additional problemsdue to the melting point of some materials used in conventionaltransducers and because the strength of most materials diminish at hightemperatures. Therefore, there exists a need for novel arrangements ofapparatus and novel methods for using such apparatus to measurerelatively high pressures at high temperatures with small diametertransducers.

SUMMARY OF THE INVENTION

Apparatus, methods, and other embodiments associated with measuringenvironmental parameters are described herein. In one embodiment, atransducer comprises a tube, an elongated member, a first reflectivesurface, a second reflective surface, and an optical fiber. The tube hasa first end and a second end, and the elongated member also has a firstend and a second end, with the first end of the elongated member securedto the tube. The second reflective surface is secured to the second endof the elongated member, and the first reflective surface is spacedapart from the second reflective surface and secured to the second endof the tube. The optical fiber is positioned to direct light towards thefirst and second reflective surfaces and to collect the reflected lightfrom these two surfaces. As the pressure exerted on the transducerchanges, the gap between the two reflective surfaces changes. The tworeflective surfaces comprise an interferometric sensor, and the lightreflected from these two surfaces may be interrogated to determine theprecise gap between the two surfaces at any pressure. By calibrating thetransducer at known pressures and temperatures, one can determine theprecise pressure or temperature for any measured gap.

DESCRIPTION OF THE DRAWINGS

Operation of the invention may be better understood by reference to thefollowing detailed description taken in connection with the followingillustrations, wherein:

FIG. 1 is a schematic illustration of an interferometric transducer tomeasure environmental parameters.

FIG. 2 is a schematic illustration of a Fabry-Perot interferometersensor for use with a transducer.

FIG. 3 is a cross-sectional illustration of a portion of aninterferometric transducer that defines a graded glass to metal seal.

FIG. 4 is a schematic illustration of a transducer that includeshydrogen getters and vacuum seal for measuring absolute pressure.

FIG. 5 is a schematic illustration of an interferometric transducer tomeasure environmental parameters according to one embodiment of theinvention.

FIG. 6 is a schematic illustration of a detail view of theinterferometric transducer for measuring environmental parametersaccording to FIG. 5.

DETAILED DESCRIPTION

While the invention is described herein with reference to a number ofembodiments and uses, it should be clear that the invention should notbe limited to such embodiments or uses. The description of theembodiments and uses herein are illustrative only and should not limitthe scope of the invention as claimed.

Apparatus for measuring high pressure at high temperatures and methodsof using such apparatus may be arranged such that the environmentalparameters being measured do not damage the sensing apparatus or causethe sensing apparatus to inaccurately measure the environmentalparameter. For example, when measuring high pressures with a transducer,the transducer may be arranged such that forces applied by the highpressure being measured do not damage or otherwise negatively affect thesensing components measuring the pressure. In an embodiment, thecomponents that measure the high pressure do not directly bear theforces applied by the high pressures; however, the components arearranged such that the pressure affects the components, and thoseeffects may be quantified to accurately measure the pressure.

Similarly, when measuring a high temperature with a transducer, thetransducer may be arranged such that the temperature does not causeundue stress that may damage or otherwise negatively affect the sensingcomponents that measure the temperature. In an embodiment, a transduceris arranged such that a series of certain components are selected suchthat the rate of thermal expansion of each of the components is similar,and bonds between such components are not compromised by thermalstresses at high temperatures. In addition, transducers may be arrangedsuch that the transducer may measure a high pressure in an environmentthat is also subject to high temperature. In an embodiment, thecomponents that measure the high pressure are arranged such that thermalexpansion of the components is similar, and bonds between suchcomponents are not compromised at high temperatures, thus resulting inaccurate measurements of the high pressure.

In an embodiment of a transducer as described herein, the sensingcomponents may include fiber optic sensing components such as, forexample, optical fibers, Fabry-Perot interferometric sensors, and thelike. Fiber optic sensing components may be well suited for use withtransducers as described herein because such components can typicallywithstand high temperatures and harsh environments and are not generallyaffected by electromagnetic interference.

Apparatus and methods of arranging reflective surfaces and measurementof light reflected from those reflective surfaces are described in U.S.patent application Ser. No. 11/377,050, to Gibler, et al., and entitled“High Intensity Fabry-Perot Sensor,” U.S. patent application Ser. No.12/365,700, now issued as U.S. Pat. No. 7,782,465, to Gibler, et al.,entitled “High Intensity Fabry-Perot Sensor,” and U.S. patentapplication Ser. No. 12/862,635 to Gibler, et al., filed Aug. 24, 2010,both of which are incorporated herein by reference for all purposes.

An exemplary embodiment of a transducer 10 arranged to measure highpressure is schematically illustrated in FIG. 1. Generally, thetransducer 10 is arranged such that the components that measure the highpressure are shielded from that high pressure by a high strengthcomponent, such as a tube 12. In the illustrative embodiment shown inFIG. 1, the tube 12 includes an external surface 14 and an internalcavity 16. The tube 12 is positioned within a housing 18 that forms anannular cavity 20 around the tube 12. When the transducer 10 ispositioned in a high-pressure environment, such as at the bottom of anoil well, the exterior surface 14 of the tube 12 bears the full force ofthe high pressure, and any components positioned or located in theinternal cavity 16 of the tube 12 do not directly bear the force of thehigh pressure. In addition to being fabricated from a high strengthmaterial, the tube 12 also includes relatively thick walls to withstandthe forces of the applied pressure.

A fluid inlet 22 allows fluid from the surrounding environment to enterthe annular cavity 20 and apply pressure to the exterior surface 14 ofthe tube 12. For example, when a transducer 10 is lowered to the bottomof an oil well, oil flows through the inlet 22 and into the annularcavity 20 surrounding the tube 12 and applies pressure to the exterior14 of the tube 12 that is equal to the pressure at the bottom of thatoil well.

Exemplary components positioned within the interior cavity 16 of thetube 12 include: an elongated member 24, a first reflective surface 26,a second reflective surface 28, and an optical fiber 30. The elongatedmember 24 may be a pin with an elongated body 32 and a flattened headportion 34. The head 34 of the pin 24 may be secured to a first end 36of the tube 12 and the body 32 of the pin 24 may extend away from thefirst end 36 of the tube 12 towards a second and opposing end 38 of thetube 12. The head portion 34 of the pin 24 may be secured to the firstend 36 of the tube 12 by welding to promote proper alignment andpositioning of the elongated body 32 of the pin 24 with respect to thetube 12. Although the pin 24 is described and shown herein as welded tothe tube 12, it will be readily understood by those skilled in the artthat any number of securing methods may be used to secure a pin to atube. For example, a pin may be secured to the tube with adhesive bonds,a pin may be integrally formed with the tube, a pin may be mechanicallyfastened to the tube, the cavity may be machined using electrostaticdischarge machining methods, and the like.

The second reflective surface 28 may be incorporated or secured to acomponent 52, such as a support member or substrate, secured to an end40 of the pin 24 located closest to the second end 38 of the tube 12.The substrate 52 may be positioned such that the second reflectivesurface 28 is perpendicular or nearly perpendicular to the length of theelongated body 32 of the pin 24. In an alternative embodiment, thesecond reflective surface 28 may be incorporated into the end 40 of thepin 24. For example, the end 40 of the pin 24 may be polished to form areflective surface, or a reflective surface may be otherwise formed inthe end 40 of the pin 24 during fabrication.

The first reflective surface 26 may be directly or indirectly secured tothe second end 38 of the tube 12 and positioned to be parallel andspaced apart from the second reflective surface 28. Such positioning ofthe reflective surfaces 26, 28 forms a gap 42 between the reflectivesurfaces 26, 28. In one embodiment, an annular ring 44, which may bemade of glass, metal, or other such material, is secured to the secondend 38 of the tube 12. A glass window 46 with a tapered surface 54 andan opposing non-tapered surface is secured to the annular ring 44. Thenon-tapered surface may be polished or coated such that it forms thefirst reflective surface 26. Once the non-tapered surface is polished orcoated, the window 46 may be secured to the annular ring 44. In such anarrangement, the first reflective surface 26 may be positioned parallelto and spaced apart from the second reflective surface 28 to form thegap 42 between the surfaces 26, 28. The optical fiber 30 may bepositioned proximate to the first reflective surface 26. The opticalfiber 30 may be arranged to direct light at the first and secondreflective surfaces 26, 28 and receive light that is reflected back bythe first and second reflective surfaces 26, 28. Such a gap 42 may forma Fabry-Perot interferometer sensor.

The first reflective surface 26 is partially reflective. That is thesurface 26 will reflect a portion of the light directed to it by theoptical fiber 30 and allow a portion of the light to pass through thesurface 26 and on to the second reflective surface 28. The portion ofthe light reflected by the first reflective surface 26 is reflected backinto the optical fiber 30. The second reflective surface 28 may bearranged to be 100% reflective or partially reflective. The lightreflected from the second reflective surface 28 is reflected backthrough the first reflective surface 26 and into the optical fiber 30.The light reflected from the reflective surfaces 26, 28 and received bythe optical fiber 30 may be measured by computerized equipment 48 toquantitatively determine the value or length of the gap 42. Thecomputerized equipment 48 may be positioned relatively near thetransducer 10 or may be positioned at great distance from the transducer10. For example, a transducer 10 located in an oil well may relayoptical signals several thousand feet to computerized equipment 48located on the earth's surface above the oil well. As will be describedbelow, by measuring the value of the gap 42, the value of the pressurein the reservoir exerted on the transducer 10 may be determined.

In the embodiment illustrated in FIG. 1, a cap 50 is secured to thefirst end 36 of the tube 12. The cap 50 is arranged to protect the headportion 34 of the pin 24 from the high forces applied to the fluid inthe annular cavity 20. Similar to the tube 12, the cap 50 is fabricatedfrom a high strength material and includes relatively thick walls towithstand the forces applied by the surrounding pressure. The cap 50 mayinclude a cavity 51 positioned proximate to where the head 34 portion ofthe pin 24 is secured to the tube 12. The cavity 51 further insures thatthe head portion 34 is protected from high pressures by avoiding contactwith the cap 50 at the location where the head 34 is secured to the tube12.

The tube 12 and cap 50 may be generally cylindrical. The transducer 10may be arranged such that when high pressures are applied to theexternal surfaces 14 of the tube 12 and cap 50, the physical dimensionsof the tube 12 change in response to the high pressure. Thesedimensional changes to the tube 12 are relayed to the pin 24 to causethe end 40 of the pin 24 securing the second reflective surface 28 tomove and thus change the value or length of the gap 42 between thereflective surfaces 26, 28. For example, if a transducer 10 is locatedat the bottom of an oil well, oil may flow into the annular cavity 20through the inlet 22 and apply a hydrostatic pressure to the externalsurface 14 of the tube 12. Such pressure subjects the cylindrical tube12 to radial compressive forces substantially equal to the surface areaof the tube 12 multiplied by the pressure. In addition, this hydrostaticpressure also applies a force on the cap 50, which translates thepressure as a longitudinal compressive force on the tube 12 that issubstantially equal to the cross-sectional area of the tube 12multiplied by the pressure. The cavity 51 in the cap 50 positionedproximate to the location of the pin's 24 attachment to the tube 12further insures that any force on the cap 50 is translated as alongitudinal compressive force on the tube 12.

The radial forces generally result in an elongation of the tube 12 inproportion with the Poisson ratio for the material of the tube 12.Elongation of the tube 12 generally results in an increase in the gap 42between the reflective surfaces 26, 28. The elongation of tube 12 causesthe second reflective surface 28 secured to the end 40 of the pin 24 tomove, because the head portion 34 of the pin 24 is attached to the firstend 36 of the tube 12. Furthermore, since the first reflective surface26 is secured to the second end 38 of the tube 12, it will be understoodthat the gap 42 between the reflective surfaces 26, 28 increases as thetube 12 elongates. The compressive forces generally result in acompression of the tube 12 in proportion to Young's modulus for thematerial of the tube 12; therefore, the compressive forces typicallycause the gap 42 to decrease. As will be understood, the net change inthe length of the tube 12, and therefore the net change of the gap 42,may be either positive or negative depending on the pressure, propertiesof the material of the tube 12, and general dimensions of the transducer10. For a given pressure the amount of deflection is a function of thelength of the tube and not its diameter. It is therefore possible todesign a family of transducers with the same diameter, and that diametermay be relatively small.

As also will be understood, design calculations may be performed toassociate any gap length 42 with a pressure. By knowing the materialproperties of the tube 12—i.e., Poisson ratio and Young's modulus forthe material from which the tube 12 is fabricated—and the physicaldimensions of the transducer 10—the length, exterior diameter, andinterior diameter of the tube 12 and the gap length 42 at an ambientpressure, and the gap at zero pressure—a gap length 42 may be calculatedfor any pressure applied to the transducer 10.

Such a transducer 10 may be subjected to a high-pressure environment andlight may be provided from the optical fiber 30 to the first and secondreflective surfaces 26, 28. Two interfering light signals may bereflected back into the optical fiber 30 from the reflective surfaces26, 28. The interfering light signals may be channeled through theoptical fiber 30 to the computerized equipment 48, where the interferingsignals may be analyzed to calculate the actual gap length 42. Once thegap length 42 is calculated, that length 42 may be translated into avalue for the pressure being exerted on the transducer 10 based on priorcalibration data or by using the design calculations.

In one embodiment, a transducer that is 1.5 inches long and made of ahigh strength alloy, such as Inconel alloy 718 (Inconel-718), deflectsabout 10 micrometers for an applied pressure of 20,000 pounds per squareinch (“psi”). Such deflections may be measured to within 0.01% providingprecise measurements of gaps and applied pressure from the lightreflected by reflective surfaces 26, 28 of the Fabry-Perotinterferometer sensor.

Optionally heat-treating the material from which the transducer 10 isfabricated may improve the stability of the transducer 10. For example,heat-treating a transducer 10 may reduce long-term drift of a transducer10 subjected to high pressures for an extended period of time. In theembodiment of a transducer 10 fabricated from Inconel-718, thetransducer 10 may be solution annealed after welding and age hardened toform a fine grain structure with high strength properties. Such anarrangement may withstand stress of 180,000 psi. Even in the solutionannealed stage, Inconel-718 transducer 10 may withstand stress of150,000 psi.

In an embodiment, the transducer 10 may be fabricated from an alloy or aglass such as, for example, Inconel-718 alloy, Hastelloy, borosilicateglass, or leaded glass. Indeed, the transducer 10 may be fabricated fromany number of materials and should not be deemed as limited to anyspecific material or combination of materials.

In another embodiment, the transducer 10 illustrated in FIG. 1 may alsobe arranged to measure temperature. In such an embodiment, the elongatedbody or pin 24 may be fabricated from a material that has a differentcoefficient of thermal expansion than the material used to fabricate thetube 12. As such a transducer 10 is exposed to changes in temperature,the pin 24 and tube 12 will expand and contract at different rates.Similar to the description for pressure, design calculations may beperformed to associate any gap length 42 with a temperature. By knowingthe material properties of the tube 12 and pin 24—i.e., the coefficientsof thermal expansion—and the physical relationships of the components ofthe transducer 10—the lengths of the pin 24 and tube 12 and the gaplength 42 at an ambient temperature—a gap length 42 may be calculatedfor any temperature to which the transducer 10 is exposed. Transducer 10may be exposed to an elevated temperature and light may be provided fromthe optical fiber 30 to the first and second reflective surfaces 26, 28.Two interfering light signals may be reflected back into the opticalfiber 30 from the reflective surfaces 26, 28. The interfering lightsignals may be channeled through the optical fiber 30 to thecomputerized equipment 48, where the interfering signals may be analyzedto calculate the actual gap length 42. Once the gap length 42 iscalculated, that length 42 may be translated into a value for thetemperature to which the transducer 10 is exposed based on priorcalibrations or design calculations.

In another embodiment, a portion of a transducer 10 is schematicallyillustrated in FIG. 2. The tapered window 46 is bonded to the second end38 of the tube 12 through an annular ring 44, which is secured to thesecond end 38. The annular ring 44 may be fabricated from a glass with arelatively large coefficient of thermal expansion such as, for example,Schott D-263 glass, leaded glass, or borosilicate glass. In onealternative, the annular ring 44 may be fabricated from a metal withrelatively low coefficient of thermal expansion such as Kovar, which hasa thermal expansion lower than Inconel-718, which may be used tofabricate the tube.

As shown in FIG. 2, a substrate 52 with a polished glass surface isbonded to the free end 40 of the pin 24. The Fabry-Perot sensor gap 42is formed between two reflective surfaces. The first reflective surface26 is the surface of the tapered window 46 that is bonded to the annularring 44. The second reflective surface 28 is the polished glass surfaceof the substrate 52 coupled to the free end 40 of the pin 24. In such anembodiment, the material that forms the reflective surfaces may havesimilar and relatively large coefficients of thermal expansion. Someexamples of such materials include, but are not limited to, Schott D-263glass, leaded glass, or borosilicate glass. The reflective surfaces 26,28 may be polished to a flatness that is better than λ/10 and an opticalsurface finish standard of scratch/dig of 60/40. As shown in FIG. 2, thesecond end 38 of the tube includes an inner aperture to allow the end 40of the pin 24 to move freely along the longitudinal axis of pin 24. Thelength of the gap 42 may vary depending on the how the transducer willbe used. For example, for long-range applications using a single modeoptical fiber, a gap length of 80 micrometers may be appropriate. Inanother example, for short-range applications using multimode opticalfibers, a gap length of 20 micrometers may be appropriate.

FIG. 3 schematically illustrates a cross-sectional view of an exemplaryarrangement for achieving a low thermal stress joint and a secure bondbetween the annular ring 44 and the tube 12. The arrangement includes aseries of metal alloys, where each metal alloy may have a differentcoefficient of thermal expansion. The metal alloys are welded in series,with each successive metal having a lower coefficient of thermalexpansion to reduce or eliminate thermal stress between the annular ring44 and the tube 12 during large changes in temperature. For example, thematerial with the largest coefficient of thermal expansion is the tube12, which may be fabricated from Inconel-718. A first ring 64 isfabricated from Hastelloy C276, and is welded to the tube 12; a secondring 66 is fabricated from alloy 52, and is welded to the first ring 64;and a third ring 68 is fabricated from alloy 48, and is welded to thesecond ring 66. The annular ring 44 is bonded to the third ring 68 andthe tapered window 46 is bonded to the annular ring 44. Alloy 48 andalloy 52 are alloys with different relative concentrations of iron andnickel. By defining the length and selection of materials used in thepin 24 and the tube 12, the thermal sensitivity of the transducer 10 maybe such that the transducer 10 is insensitive or highly sensitive totemperature changes.

A reflective dielectric coating may be applied to the surface of thetapered window 46 nearest the pin 24 to form the first reflectivesurface 26. A reflective dielectric coating may also be applied to thepolished glass surface of the substrate 52 bonded to the pin 24 to formthe second reflective surface 28. For long-range, single modeapplications, a highly reflective coating, i.e., 85% reflectance, may beutilized. For short-range, multimode applications a less reflectivecoating, i.e., 35% may be utilized. The tapered window 46 may befabricated from Schott D-263 glass, leaded glass, or borosilicate glass.As shown in FIG. 2, the unbonded surface 54 of the tapered window 46 maybe arranged at an angle to eliminate unwanted reflections from thatsurface 54 of the window 46, which is located closest to the opticalfiber 30. The angled surface 54 prevents such unwanted reflections fromreentering the optical fiber 30 and interacting with the pair ofinterfering reflected signals. In one embodiment, the angled surface 54may be arranged at an angle that is greater than 6 degrees.

In the embodiment illustrated in FIG. 2, the Fabry-Perot gap is formedwith a structure that is subject to very low thermal stress. Inaddition, the dielectric interferometer is not subject to oxidation anddoes not suffer from issues of drift and degradation over time that maybe attributed to the oxidation of metal structures.

As described in U.S. patent application Ser. No. 11/377,050, a ball lens56 may be positioned at the end of the optical fiber 30. The ball lens56 may be used to deliver light to the reflective surfaces 26, 28 bycollimating the light directed to those surfaces 26, 28. The ball lens56 may be fused to the end of the optical fiber 30 or may be a separatecomponent aligned with the optical fiber 30. A ball lens 56 may be fusedto the end of the optical fiber 30 by heating the optical fiber 30 toits melting point, where surface tension produces a sphere oftransparent silica, which forms the ball lens 56 upon cooling. Such aheating and cooling process inherently centers the ball lens 56 on thelongitudinal axis of the optical fiber 30. In one embodiment, thediameter of the ball lens 56 is approximately 340 micrometers.

As described in U.S. patent application Ser. No. 11/377,050, a ball andsocket assembly 58 may be used to better align the optical fiber 30 andthe ball lens 56. A ferrule 60 is positioned within the ball and socketassembly 58 and holds the optical fiber 30 such that the ball 62 may berotated to align the optical fiber 30 as desired. The ball 62 may alsobe slid laterally to position the ball lens 56 relative to the taperedwindow 46. In another embodiment, alignment of the optical fiber 30 maybe accomplished by polishing the end of the structure that supports theball lens 56 at an angle that assures the light beam is perpendicular tothe first and second reflective surfaces 26, 28.

FIG. 4 illustrates a transducer 10 that includes a pair of hydrogengetters 70 that absorb hydrogen to maintain a high stability (low drift)of the transducer 10. A getter 70 is arranged to absorb hydrogen ionsand molecules diffusing through the transducer 10 when the transducer 10is set to measure absolute pressure. To measure absolute pressure, thecavity 74 inside the transducer body is evacuated and sealed 76.Absorption of hydrogen by the getter 70 decreases or eliminates theprobability that stray hydrogen molecules will increase the pressureinside the evacuated transducer 10 and lead to inaccurate pressurereadings over time. An increase in hydrogen partial pressure may alsochange the thermal sensitivity of the transducer 10 and result in a lossof calibration of the transducer 10 over time. The getters 70 may bepositioned at the end of the cap or behind the ball lens assembly. Thegetters 70 may be sized for the expected service life of the transducer10

Alternatively, the transducer 10 may function without the getters 70 ifthe Fabry-Perot gap 42 is vented to atmospheric pressure and thetransducer 10 is designed for measuring gage pressure. In such anembodiment, any hydrogen that diffuses into the transducer 10 willescape into the atmosphere and will not affect the length of the gap.

In another embodiment, the pressure inlet 22 may be positioned adjacentto the end cap 50 as shown in FIG. 1, and an external pressure isolationstep 72 shown in FIG. 4 may be positioned to the left of the ball lens56 (with respect to FIG. 4) to cause the transducer 10 to be insensitiveto applied external pressure,

The pressure transducer 10 may also be affected by thermal sensitivityand may require a temperature measurement or thermal correction toensure precise and accurate measurements. For example, a temperaturesensor may be inserted behind the ball lens 56 to accurately measure thetemperature of the transducer 10. The signal processor may thus becorrected for known thermal sensitivity of the pressure sensor.

FIG. 5 is a schematic illustration of an interferometric transducer tomeasure environmental parameters according to one embodiment of theinvention. The exemplary embodiment shown is a pressure transducer andcan be modified, for example, for a temperature transducer. Thetransducer main body 8, which can include various sensors, electronics,etc., not described herein, is attached to the housing 18. The housing18 is preferably cylindrical, defining an interior cavity 20. Thehousing is preferably made of a high strength material given theenvironment and use to which it is exposed. The transducer main body 8can also be attached directly to the tube 12. The tube 12 is positionedin the interior cavity 20 of the housing, such that interior cavity 20is annular. The tube 12 is also preferably cylindrical and defines aninterior cavity 16. The elongated member 32 is positioned inside theinterior cavity 16, such that the cavity 16 is annular as well. At thefirst end 36 of the tube 12 is cap 50 which is attached to the first endof the tube and seals the interior cavity 16. In use as a pressuretransducer, environmental fluid is allowed to enter the annular cavity20 between the housing 18 and tube 12. Fluid is not allowed to flow fromthe environment into the annular cavity 16 between the elongated member32 and the tube 12. Annular cavity 16 can be evacuated and sealed. Thecap 50 is attached to the tube 12 and spaced apart from the elongatedmember 32 with enough clearance at gap 51 so that the cap 50 does notcontact the elongated member, even upon deflection, expansion, or otherdistortion from exposure to the environment or during use. The cap 50includes an inlet to allow pressure communication from outside thetransducer to the interior cavity 20. The first end 34 of the elongatedmember 32 is attached to the first end 36 of the tube 12. Preferably thehousing, tube 12 and elongated member 32 are made of metal. Attachmentsof metal parts are preferably by welding.

Window 46, typically made of glass, as explained above, is attached tothe second end 38 of the tube 12. The window 46 includes the firstreflective surface 26. The window 46 is typically attached to the secondend of the tube by bonding. Similarly, the second reflective surface 28is attached to the second end 40 of the pin 32, a gap 42 defined betweenthe reflective surfaces. The second reflective surface 28 is, in apreferred embodiment, a surface of substrate 52, which is a section ofthe tube 12.

Optical assembly 31 is positioned at one end of the tube 12. In apreferred embodiment, the optical assembly includes an optical fiber 30,a lens 56, ferrule 60, an adjustment subassembly 59, which can comprisea ball-and-socket assembly 58 as shown, a mounting sleeve 33, etc. Themounting sleeve 33 is attached to the tube 12, preferably by welding.The optical assembly, adjustment subassembly, and operation of theinterferometric sensor are described above and in U.S. patentapplication Ser. No. 11/377,050, to Gibler, et al., and entitled “HighIntensity Fabry-Perot Sensor,” U.S. Ser. No. 12/365,700 (now issued asU.S. Pat. No. 7,782,465), to Gibler, et al., entitled “High IntensityFabry-Perot Sensor,” and U.S. Ser. No. 12/862,635 to Gibler, et al.,filed Aug. 24, 2010, which are incorporated herein by reference for allpurposes.

The tube 12 can be made of multiple tube sections, each of differentmaterial. In the preferred embodiment shown in FIG. 5, the tube 12 iscomprised of a first tube section 12 a and a second tube section 12 b.First tube section 12 a makes up the majority of the tube 12 and is madeof a high strength material to withstand the extreme environmentsencountered in use. For example, tube section 12 a can be made ofInconel-718. Most high strength materials, however, have highcoefficients of thermal expansion (CTE). The second end 38 of the tube12, however, is bonded to window 46, which is made of glass typically,and therefore has a relatively low CTE. To achieve a low stress thermaljoint, tube section 12 b is made of a material having a lower CTE thanthat of tube section 12 a. (FIG. 3 herein and accompanying text describea tube 12 having multiple sections or rings, including annular ring 44,first ring 64, second ring 66 and third ring 68.) The tube 12 can bemade of multiple sections of varying material, but in the preferredembodiment seen in FIG. 5, the tube 12 is made of two sections, 12 a and12 b. In a preferred embodiment, first tube section 12 a is made ofInconel-718, having a CTE of 8.0×10⁻⁶ in/in/F, and second tube section12 b is made of Hastelloy C276, having a CTE of 7.4×10−6 in/in/F. Wherethe tube sections are both metal, as preferred, they are welded togetherat their seam.

In addition to providing a lower stress thermal bond, making the tube 12of sections of different material provides another advantage. Selectingappropriate materials for, and relative longitudinal lengths of, thetube sections allows the thermal sensitivity of the composite tube 12 tobe tuned for optimal thermal sensitivity which is not possible where thetube is of a single material. For example, in a pressure transducer, thematerials and lengths are selected to minimize thermal sensitivity whilestill providing the strength and other properties necessary. In atemperature transducer, the materials are preferably selected tomaximize thermal sensitivity.

Note that the majority of the length of the tube 12 (along a majority ofthe length of the first section of the tube 12 a) defines, in part, theannular cavity 20, and is open to the high hydrostatic pressure in thecavity during use. However, the second section 12 b of the tube 12 isisolated from the high-hydrostatic pressure and not in fluidcommunication with cavity 20.

Similar to the tube, the elongated member 32 is made of multiplesections in the preferred embodiment seen in FIG. 5. The elongatedmember 32 is comprised of three sections. First elongated member section32 a makes up the majority of the elongated member and is preferably ofa metal or material of high strength. Preferably, the first elongatedmember section 32 a is of the same material as first tube section 12 a.Second elongated member section 32 b is typically smaller than firstsection 32 a and is made of a different material having a lower CTE thanthat of the first section 32 a. Third elongated member section 32 c ispreferably made of glass (and may be thought of as corresponding tosubstrate 52 in FIG. 2 above). Elongated member 32 is preferablycomprised of multiple sections of differing material for the samereasons as explained above with respect to sections of tube 12. Adjacentsections are preferably welded together where both are metal and bondedwhere one section is metal and another glass.

In the preferred embodiment, the overall CTE of the elongated member 32is designed to match the overall CTE of the tube 12. The elongatedmember 32 and tube 12 are of comparable, although typically notidentical lengths. Additionally, in the embodiment shown, the elongatedmember is made of three sections of differing material and the tube ismade of two sections of differing material. Consequently, the firstsection 12 a of the tube 12 is not identical in length to the firstsection 32 a of the elongated member 32. Similarly, the second sections12 b and 32 b of the tube 21 and elongated member 32, respectively, arenot of identical lengths in the preferred embodiments.

FIG. 6 is a schematic illustration of a detail view of theinterferometric transducer for measuring environmental parametersaccording to FIG. 5. Like numbers are used throughout. The pressuretransducer seen in FIGS. 5-6 can be modified for other purposes, such asfor a temperature transducer. For example, the relative lengths of thehigh-CTE and low-CTE sections of the tube (and elongated member) may bechanged to maximize the thermal sensitivity of the sensor in atemperature transducer. Persons of skill in the art will recognize thedesign modifications necessary and preferable.

The invention has been described above and, obviously, modifications andalternations will occur to others upon a reading and understanding ofthis specification. The claims as follows are intended to include allmodifications and alterations insofar as they come within the scope ofthe claims or the equivalent thereof.

1. A transducer for measuring environmental parameters, the transducercomprising: a housing having an interior surface; a tube positionedwithin the housing and defining a first cavity between the housing andthe tube, the tube having a first end and a second end: an elongatedmember positioned within the tube and defining a second cavity betweenthe tube and the elongated member, the elongated member having a firstend and a second end, wherein the elongated member is secured to thetube; a first reflective surface and a second reflective surface, wherethe second reflective surface is positioned proximate to the second endof the elongated member and the first reflective surface is spaced apartfrom the second reflective surface; and an optical fiber positioned todirect light towards the first and second reflective surfaces.
 2. Atransducer as in claim 1, wherein the environmental parameter measuredby the transducer is pressure.
 3. A transducer as in claim 2, whereinthe first cavity is in fluid communication with an environmental fluid.4. A transducer as in claim 3, wherein the second cavity is sealed fromthe environmental fluid.
 5. A transducer as in claim 3, furthercomprising a cap attached to one end of the tube, the cap allowingcommunication of pressure from the exterior to the interior of the tube.6. A transducer as in claim 1, further comprising a window attached toone end of the tube, a surface of the window defining the firstreflective surface.
 7. A transducer as in claim 1, wherein the tube ismade of at least a first and second tube section, and wherein the firsttube section is made of a material having a first coefficient of thermalexpansion, and the second tube section is made of a material having asecond coefficient of thermal expansion.
 8. A transducer as in claim 6,wherein the tube is made of at least a first and second tube section,and wherein the first tube section is made of a first material having afirst coefficient of thermal expansion and the second tube section ismade of a second material having a second coefficient of thermalexpansion lower than the first coefficient of thermal expansion, and thewindow attached at the second tube section.
 9. A transducer as in claim7, wherein the materials and relative lengths of the first and secondtube sections are selected to optimize thermal sensitivity for use ofthe transducer.
 10. A transducer as in claim 9, wherein theenvironmental parameter to be measured is temperature, and wherein thematerials and relative lengths of the tube sections are selected tomaximize thermal sensitivity.
 11. A transducer as in claim 7, whereinthe environmental parameter to be measured is pressure, and wherein thefirst tube section is exposed to environmental hydrostatic pressure, andwherein the second tube section is isolated from the environmentalhydrostatic pressure.
 12. A transducer as in claim 1, wherein theelongated member is made of at least a first and second member section,and wherein the first member section is made of a first material havinga first coefficient of thermal expansion, and the second member sectionis made of a second material having a second coefficient of thermalexpansion lower than the first coefficient of thermal expansion.
 13. Atransducer as in claim 12, the elongated member having a third membersection having a third coefficient of thermal sensitivity lower than thesecond coefficient of thermal expansion, and wherein the secondreflective surface is defined by a portion of the third member section.14. A transducer as in claim 13, wherein the environmental parameter tobe measured is temperature, and wherein the materials and relativelengths of the tube sections are selected to maximize thermalsensitivity.
 15. A transducer as in claim 8, wherein the elongatedmember is made of at least a first and second member section, andwherein the first member section is made of a first material having afirst coefficient of thermal expansion, and the second member section ismade of a second material having a second coefficient of thermalexpansion lower than the first coefficient of thermal expansion.
 16. Atransducer as in claim 15, the elongated member having a third membersection with a third coefficient of thermal sensitivity which is lowerthan the second coefficient of thermal expansion, and wherein the secondreflective surface is defined by a portion of the third member section.17. A transducer as in claim 15, wherein the environmental parameter tobe measured is temperature, and wherein the materials and relativelengths of the tube sections are selected to maximize thermalsensitivity.
 18. A transducer as in claim 16, wherein the materials andrelative lengths of the member sections are selected to optimize thermalsensitivity.
 19. A transducer as in claim 15, and wherein the overallcoefficient of thermal expansion of the tube matches the overallcoefficient of thermal expansion of the elongated member.
 20. Atransducer as in claim 16, and wherein the overall coefficient ofthermal expansion of the tube matches the overall coefficient of thermalexpansion of the elongated member.